RT8487. High Efficiency BCM LED Driver Controller for High Power Factor Offline Applications. General Description. Features. Ordering Information

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1 High Efficiency BCM LED Driver Controller for High Power Factor Offline Applications General Description The RT8487 is a Boundary mode high PF floating buck constant LED current output controller with an internal gate driver. The RT8487 features a ZCS detector which keeps system operating in BCM and obtaining excellent power efficiency, better EMI performance. The RT8487 achieves high power factor correction (PFC) and low total harmonic distortion of current (THDi) by a smart internal line voltage compensation circuit which has minimized system component counts; saved both PCB size and total system cost. Especially, the RT8487 can use a cheap simple drum core inductor in the system instead of an EE core to obtain high efficiency. The RT8487 is housed in a TSOT-23-6 package. Thus, the components in the whole LED driver system can be made very compact. Ordering Information RT8487 Note : Richtek products are : Package Type J6 : TSOT-23-6 Lead Plating System G : Green (Halogen Free and Pb Free) RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Features Support High Power Factor and THDi Consideration Applications Programmable Constant LED Current with Highly Precision Current Regulation Extremely Low Quiescent Current Consumption and 1µA Shutdown Current True Low System BOM Cost and Economical Floating Buck Converter Solution Unique Programmable AND Pin for ZVS Setting to Achieve Best Power Efficiency Universal Input Voltage Range with Off-Line Topology Built-in Over Thermal Protection Built-in Over Voltage Protection Output LED String Open Protection Output LED String Short Protection Output LED String Over Current Protection Applications E27, PAR, Light Bar, Offline LED Lights Pin Configurations (TOP VIEW) SENSE VC AND VCC GND GATE TSOT-23-6 Marking Information 00=DNN 00= : Product Code DNN : Date Code 1

2 Simplified Application Circuit Buck type: Bridge Rectifier + - R1 R2 C IN AND VCC RT8487 VC GATE Q1 C1 R3B C2 GND SENSE D2 Bootstrap Diode R S L1 D1 C OUT DS March

3 Functional Pin Description Pin No. Pin Name Pin Function 1 VCC 2 GND Ground. RT8487 Supply Voltage Input. For good bypass, a ceramic capacitor near the VCC pin is required. 3 GATE Gate Driver Output for External MOSFET Switch. 4 AND AND Function Pin. 5 VC Close Loop Compensation Node. 6 SENSE Function Block Diagram LED Current Sense Input. The typical sensing threshold is 250mV between the SENSE and GND pin. VCC Regulator SENSE + EA - State Machine A GATE V 250mV GND VC AND Operation The RT8487 senses true average output current and keeps the system driving constant output current. The VC pin is the compensation node in this close loop system and dominates the frequency response. To stabilize the system and achieve better PFC / THDi, proper selection of a compensation network is needed. 3

4 Absolute Maximum Ratings (Note 1) Supply Input Voltage (VCC) V Power Dissipation, TA = 25C TSOT W Package Thermal Resistance (Note 2) TSOT-23-6, JA C/W Lead Temperature (Soldering, 10 sec.) C Junction Temperature C Storage Temperature Range C to 150C ESD Susceptibility (Note 3) HBM (Human Body Model) kV MM (Machine Model) V Recommended Operating Conditions (Note 4) Supply Input Voltage, VCC V to 30V Ambient Temperature Range C to 85C Junction Temperature Range C to 125C Electrical Characteristics (V CC = 24V, TA = 25C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit VCC UVLO ON VUVLO_ON V VCC UVLO OFF VUVLO_OFF V VCC Shutdown Current ISD VCC = VUVLO_ON 3V A VCC Quiescent Current IQC Gate stands still ma VCC Operating Current ICC By CGATE = 1nF, Freq.= 20kHz ma VCC OVP Level VOVP V Sense Pin Leakage Current ISENSE VSENSE = 3V A Current Sense Threshold VSENSE mv AND Pin Leakage Current IAND VAND = 5V A GATE Voltage High VGATE_H IGATE = 0mA IGATE = 10mA V GATE Driver Rising Time tr Falling Time tf ns Note 1. Stresses beyond those listed Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect device reliability Note 2. JA is measured at T A = 25C on a high effective thermal conductivity four-layer test board per JEDEC Note 3. Devices are ESD sensitive. Handling precaution recommended. Note 4. The device is not guaranteed to function outside its operating conditions. DS March

5 Typical Application Circuit Buck : RT8487 Bridge Rectifier + - R1 511K C IN 0.1μF/500V R2 511K C1 4.7μF/50V R3B 100k C2 1μF AND VCC RT8487 VC GATE GND SENSE Q1 MTN4N60 R S 0.8 L1 470μH R B 10 D2 RS1M D1 ES2J C OUT 270μF/63V 5

6 Typical Operating Characteristics 2.0 Operating Current vs. Supply Voltage 2.0 Operating Current vs. Temperature Operating Current (ma) Operating Current (ma) Supply Voltage (V) GATE with 1nF 0.2 V CC = 24V, GATE with 1nF Temperature ( C) 40 OVP vs. Temperature 20 UVLO vs. Temperature OVP (V) Temperature ( C) UVLO (V) UVLO_ON UVLO_OFF Temperature ( C) 500 Sense Threshold vs. Supply Voltage 500 Sense Threshold vs. Temperature Sense Threshold (mv) Sense Threshold (mv) Supply Voltage (V) 50 V CC = 24V Temperature ( C) DS March

7 100 Efficiency vs. Input Voltage 350 Output Current vs. Input Voltage 340 Efficiency (%) V IN_AC = 90V to 264V 80 I OUT = 300mA, LED 14pcs, L = 470μH Output Current (ma) V IN_AC = 90V to 264V 260 I OUT = 300mA, LED 14pcs, L = 470μH Input Voltage (V) Input Voltage (V) 1.00 Power Factor vs. Input Voltage Input and Output Current Power Factor V IN_AC = 90V to 264V 0.75 I OUT = 300mA, LED 14pcs, L = 470μH Input Voltage (V) V IN (500V/Div) I IN (200mA/Div) V OUT (50V/Div) I OUT (500mA/Div) Time (5ms/Div) V IN_AC = 264V I OUT = 300mA, LED 14pcs, L = 470μH Power On Power Off V IN_AC = 264V V IN_AC = 264V V IN (500V/Div) V IN (500V/Div V OUT (20V/Div) I OUT = 300mA, LED 14pcs, L = 470μH V OUT (20V/Div) I OUT (200mA/Div) I OUT = 300mA, LED 14pcs, L = 470μH I OUT (200mA/Div) Time (100ms/Div) Time (100ms/Div) 7

8 50% 45% 40% 35% Class C Measured Total Harmonic Distortion V IN_AC = 115V/60Hz I OUT = 300mA, LED 14pcs, L = 470μH 50% 45% 40% 35% Class C Measured Total Harmonic Distortion V IN_AC = 230V/50Hz I OUT = 300mA, LED 14pcs, L = 470μH 30% 30% 25% 25% 20% 20% 15% 15% 10% 10% 5% 5% 0% % DS March

9 Application Information RT8487 is a boundary mode, high efficiency constant current controller with internal high side driver, which can be used in buck and buck-boost configuration, to provide a constant output current to the (LED) load. It contains special circuitry for achieving high power factor and low input current THD, while minimizing external component count. The small SOT23-6 package keeps application footprint small, and makes RT8487 a cost effective solution for off-line LED drivers. The RT8487 can achieve high accuracy LED output current via the average current feedback loop control. The internal sense voltage (250mV typ.) is used to set the average output current. The average current is set by the external resistor, RS. The sense voltage is also used for over current protection(ocp) function. The typical OCP threshold is about seven times of the sense voltage threshold. Under Voltage Lockout (UVLO) The RT8487 includes a UVLO function with 10.8V hysteresis. For system start up, the VIN must rise over 18V (typ.) to turn on the GATE terminal. The GATE terminal will turn off if VIN falls below 7.2V (typ.) Setting Average Output Current The output current that flows through the LED string is set by an external resistor, RS, which is connected between the GND and SENSE pins. The relationship between output current, IOUT, and RS is shown below : 250 I OUT = ma R S Start-Up Resistor The start-up resistor should be chosen to set the start up current exceeds certain minimum value. Otherwise, the RT8487 may latch off and the system will never start. The start-up current equals 2 90V / R1 +R2 (for 110VAC regions), and equals 2 180V / R1 +R2 (for 220VAC regions). The typical required minimum start-up current is 100A. The typical total start up resistance (R1 + R2) is around 1M Ohm for universal inputs. Input Diode Bridge Rectifier Selection The current rating of the input bridge rectifier is dependent on the VOUT /VIN conversion ratio and out LED current. The voltage rating of the input bridge rectifier, VBR, on the other hand, is only dependent on the input voltage. Thus, the VBR rating is calculated as below : V = V BR AC(MAX) where VAC(MAX) is the maximum input voltage (RMS) and the parameter 1.2 is used for safety margin. For this example : BR AC(MAX) V = V = = 448V If the input source is universal, VBR will reach 448V. In this case, a 600V, 0.5A bridge rectifier can be chosen. Input Capacitor Selection For High Power Factor application, the input Capacitor CIN should use a small value capacitance to achieve line voltage sine-wave. The voltage rating of the input filter capacitor, VCIN, should be large enough to handle the input voltage. AC(MAX) V V = = 448V IN 9

10 Thus, a 0.1F / 500V film capacitor can be chosen in this case. Inductor Selection For high power factor application, the RT8487 operates the converter in BCM (Boundary-Condition Mode). The inductance range is defined by peak current of inductor maximum and minimum value of switching on time and off time, for ensuring the inductor operates in BCM. The peak current of inductor is showed as below : 2Pin I PEAK = V PEAK F a V where a = V and Fa a 0~0.7 OUT PEAK a a a a , The inductance range is showed as below : VOUT TOFF L = = IPEAK VPEAK VOUT TON IPEAK Where 0.5s TON 35s and 2s TOFF 30s The frequency at the top of the sine wave can be calculated : 1 f SW = T ON + T OFF + T DELAY (TDELAY is determined by the resistor connected to AND pin, see Turn on delay time) Turn On Delay Time After the inductor current has reached zero, a resonance will occur between the inductor and the MOSFET drain-source capacitance. In order to minimize the MOSFET switching losses, RT8487 provides the flexibility to adjust the delay time of next switch-on cycle in order to switch-on at the maximum point of the resonance, which corresponds to the minimum drain-source voltage value. The delay time from zero current point to the maximum of the switch resonance which can be calculated from : T = L1 C resonance SW where CSW is the capacitance at the switch node, mostly determined by the MOSFET drain-source capacitance. The delay time TDELAY from zero current detection point to next MOSFET switch-on cycle can be adjusted by the resistor value R3B connected between AND pin and IC GND T DELAY (μs)=(-0.4 x R3B x R3B ) x 10-6 R3B resister value in k. Forward Diode Selection When the power switch turns off, the path for the current is through the diode connected between the switch output and ground. This forward biased diode must have minimum voltage drop and recovery time. The reverse voltage rating of the diode should be greater than the maximum input voltage and the current rating should be greater than the maximum load current. The peak voltage stress of diode is : D AC(MAX) V V = = 448V The input source is universal (VIN = 85V to 264V), VD will reach 448V. MOSFET Selection The peak current through this MOSFET will be over the maximum output current. This component current rating should be greater than 1.2 times the maximum load current and the reverse voltage rating of the MOSFET should be greater than 1.2 times the maximum input voltage, assuming a ±20% output current ripple. The peak voltage rating of the MOSFET is : Q1 AC(MAX) V = V = = 448V The largest peak current will occur at the highest VIN. The current rating of MOSFET is determined by the DS March

11 OCP threshold which is about seven times of the sense voltage threshold. Thermal Protection (OTP) A thermal protection feature is included to protect the RT8487 from excessive heat damage. When the junction temperature exceeds a threshold of 150C, the thermal protection OTP will be triggered and the GATE will be turned off. Thermal Considerations For continuous operation, do not exceed absolute Maximum Power Dissipation (W) Ambient Temperature ( C) Four-Layer PCB maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) TA) / JA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and JA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125C. The junction to ambient thermal resistance, JA, is layout dependent. For TSOT-23-6 package, the thermal resistance, JA, is 197.4C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25C can be calculated by the following formula : Figure 1. Derating Curve of Maximum Power Dissipation Layout Considerations For best performance of the RT8487, the following layout guidelines should be strictly followed. The hold up capacitor, C1, must be placed as close as possible to the VCC pin. The compensation capacitor, C2, and delay resistor, R3B, must be placed as close as possible to the VC and the AND pin. The IC GATE and GND pin are high frequency switching nodes. Both traces must be as wide and short as possible. Keep the main traces with switching current as short and wide as possible. Place CIN, L1, Q1, RS, COUT, and D1 as close to each other as possible. PD(MAX) = (125C 25C) / (197.4C/W) = 0.5W for TSOT-23-6 package The maximum power dissipation depends on the operating ambient temperature for fixed TJ(MAX) and thermal resistance, JA. The derating curve in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 11

12 Place the compensation Components C2 and R3B as close as possible to the IC V MAIN R1 R2 C2 R3B SENSE VC AND Place the MOSFET Q1, the Diode D1 and the resistor R S as close as possible to the each other C IN RT8487 VCC R B ZD(Option) C1 1 VCC Analog GND 2 3 GND GATE R G Q 1 R S SENSE L 1 Analog GND D 2 LED+ D1 C OUT Power GND LED- Place the capacitor C1 as close as possible to the VCC pin Kelvin sense from the sense resistor directly from the sense resistor is necessary to avoid the sense threshold setting error by the parasitic PCB trace resistance. Narrow trace from main circuit to the IC to avoid the switching noise Figure 2. PCB Layout Guide DS March

13 Outline Dimension Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A A B b C D e H L TSOT-23-6 Surface Mount Package Richtek Technology Corporation 14F, No. 8, Tai Yuen 1 st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863) Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. 13